Guidance mechanisms direct the migration of embryonic cells during the assembly of organs and tissues. Many of the molecules that play critical roles in these processes were initially discovered through studies of nervous system (NS) development. Recently, exciting progress has been made toward explaining neuronal patterning in terms of specific molecular mechanisms. Surprisingly, neurobiologists have discovered that a relatively small number of guidance molecules, principally netrins, Slits, semaphorins and ephrins, generate the complex topology of the NS. In general, they function by delivering precisely timed attractive or repulsive cues that establish pathways, targets and boundaries. Since the molecular mechanisms that mediate the intricacies of neuronal interactions are used during other developmental processes, we considered the possibility that formation of the human placenta might involve a subset of these molecules. Is there evidence to support this hypothesis? At a morphological level, placental development, like formation of the NS, entails intricate and precisely patterned cell-cell interactions. At a molecular level, our data show that the subpopulation of cytotrophoblasts (CTBs) that invade the uterine wall express members of two families of neuronal guidance molecules--semaphorins and ephrins. Interestingly, the expression patterns of several of these molecules, in the context of their known functions, suggest that they could regulate crucial aspects of placental development such as triggering uterine invasion and the ability of CTBs to discriminate between uterine arteries and veins. Recently completed function perturbation studies show that both semaphorins and ephrins play important roles as CTB differentiate/invade. Thus, we now propose experiments to test the following hypothesis: mechanisms that play master regulatory roles in formation of the NS are equally important components of human placental development. In this new paradigm, specific neuronal guidance molecules direct CTB movement through the uterus where stable interactions with maternal cells take place. In Aim 1, we will study the role of semaphorins in guiding CTB uterine invasion. In Aim 2, we will focus on the role of ephrins in patterning CTB interactions with maternal cells. In Aim 3, we will determine if the expression of either semaphorin or ephrin family members is dysregulated in the pregnancy complication preeclampsia (PE), which is associated with abnormally shallow CTB invasion and gestational trophoblast diseases (GTDs), in which CTBs invade beyond the inner portion of the myometrium. At the conclusion of these experiments, we will have achieved a new level of understanding of the mechanisms that control formation of the maternal-fetal interface. In doing so, we will gain important insights into the etiology of PE, GTDs and, perhaps, other types of tumors.